Controlled drug release involves a combination of a polymer matrix with bioactive drugs such that the drugs can be delivered in a predictable manner. Polymeric materials, including biodegradable synthetic polymers such as poly(D,L-lactide-co-glycolide) (PLGA) and natural polymer such as collagen and alginate have been used as drug delivery matrices. These polymer matrices function in many ways as an artificial extracellular matrix (ECM) to stabilize encapsulated proteins, such as growth factors. See Jiang et al., “Biodegradable poly(lactic-o-glycolic acid) microparticles for injectable delivery of vaccine antigens,” Adv. Drug Deliv. Rev. 57 (2005) 391-410; see also Wee et al., “Protein release from alginate matrices,” Adv. Drug Deliv. Rev. 31 (1998) 267-285.
The release of encapsulated protein drugs are controlled by both passive diffusion of protein drugs and degradation of polymer matrices. Encapsulation and controlled release are of particular importance for protein drugs with short half-lives when free in solution, and for reduced systemic toxicity. However, preservation of biological activity of incorporated protein drugs in a polymer matrix and control of subsequent release remain major challenges.
Silk fibroin has a long history in clinical applications used as suture threads, and now it is finding new and important applications in the tissue-engineering field as a scaffold support for the growth of artificial tissues such as bone and cartilage. Recently, the use of silk fibroin for controlled drug delivery has been explored with electrospun silk fiber mats that encapsulated bone morphogenetic protein 2 (BMP-2). See Li et al., “Electrospun silk-BMP-2 scaffolds for bone tissue engineering,” Biomaterials 27 (2006):3115-3124. Hoffman investigated the encapsulation and release of different proteins such as horseradish peroxidase (HRP) and lysozyme from silk films and the correlation between silk crystallinity that were induced by methanol and protein release behaviors. It was found that high silk crystallinity could significantly retard the release of encapsulated proteins. See Hofmann et al., “Silk fibroin as an organic polymer for controlled drug delivery,” J. Control Release 111 (2006):219-227.
Thus, silk fibroin holds great promise for controlled drug delivery due to its unique structure and crystallinity properties as well as the other advantages discussed above. Silk microspheres can be fabricated using physical methods such as spray-drying, however, harsh conditions such as high temperature have prohibited their uses as a protein drug delivery carrier. See Hino et al., “Change in secondary structure of silk fibroin during preparation of its microspheres by spray-drying and exposure to humid atmosphere,” J. Colloid Interface Sci. 266 (2003) 68-73. In addition, conventional microspheres typically have a large size (above 100 μm), making them less useful as encapsulation vehicles for many of the smaller drug molecules.
Accordingly, what is needed in the art is a way to prepare silk fibroin microspheres under mild conditions so that protein drugs and other therapeutic agents can be encapsulated in the microspheres and released in their active forms. This invention answers that need.